Commercial magnetic disk drive areal densities now exceed 300 Gbits/sq. in., and track densities can be greater than 250,000 tracks per inch. Advanced servo techniques are required to further improve disk drive performance. At the needed track densities, the heads (sensors) must stay centered on the narrow tracks to within a tolerance on the order of a few nanometers as the disk rotates under the heads at thousands of rpm. The servo fields, which encode positional information, are permanently written onto the disk during the manufacturing process. The servo information is processed by an electronics control system that adjusts the physical position of the actuator on which the heads are mounted.
The process by which the position information is written onto the disks is referred to as servo writing. Conventionally servo writing has been performed by a dedicated device called a servowriter that is distinct from the disk drive itself. Another approach is have the disk drive heads write the servo information, which is called self-servo writing.
Conventional magnetic disks have continuous thin films in which the magnetic transitions are recorded, but one technique for improving areal densities is to pattern the films into discrete track media (DTM) in which the tracks are formed from continuous strips (concentric rings) of magnetic material separated by small gaps where the magnetic material has been removed. More recently bit-patterned media (BPM) have been proposed that use nanometer scale magnetic islands or dots arranged in tracks on the recording surface. Concentrically patterned BPM can be considered to be interchangeable with DTM in the following specification. DTM and BPM place constraints on servo pattern design, because arbitrary features may not be compatible with the appropriate fabrication methods. Required servo patterns may be more complex than the data patterns. For example, planarization constraints might require that all pre-patterned features are constructed of grooves and lands with common dimensions (i.e., servo patterns have the same land and groove dimensions as data tracks). The planarization constraint allows grooves to vary in orientation and absolute position, provided land and groove dimensions are generally fixed.
Patterned media is typically fabricated using nanoimprint lithography (NIL), which mechanically deforms the imprint resist to create patterns. A master template is used to fabricate a plurality of stamper tools that are then used for imprinting the substrates for the patterned media. The required servo patterns must be included in the master template. One method of fabricating patterned media templates grows self-assembly structures on top of a lithographically-defined template. For example e-beam lithography can be used to pattern holes for a master mold substrate. After the e-beam patterning, block copolymer self-assembly can be used to improve the uniformity of the e-beam dots and to fill-in missing dots. Self-assembled structure on the templates imposes constraints on the features that can be formed by filling in the gaps and minimizes the energy of the system. For example, asymmetric features, such as low order Gray code bits, may be incompatible with the constraints of self-assembly structures.
The servo data on the disk provides several fundamental functions and is conventionally arranged in distinct fields that are arranged in sequence in each servo sector along the direction of the track. First, it supplies a synchronizing timing mark (known as the Servo Track Mark (STM) or Servo Address Mark (SAM)). Next is a 10-30 bit digital field, which provides an integer track-ID (TID) number and additional information to identify the physical servo sector number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is rapidly moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek. The servo field also includes a position error field, which provides the fractional-track Position Error Signal (PES). During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The conventional PES pattern is called a quad-burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerlines of adjacent tracks. The quad-burst pattern is repeated for each set of two tracks, so only local (fractional) information is provided. Each servo wedge has four (A,B,C,D) sequential slots reserved for PES bursts. Each track has a centered PES burst in only one of the four slots. Thus, when the head is centered over a selected track, it will detect the strongest signal from a burst centered on the selected track, but it will also detect a weaker signal from bursts on the adjacent tracks. For example, when the head is centered over a track with a burst in the A-position, it might also detect a subsequent weak B-burst on the adjacent track on the right and then a weak D-burst from the adjacent track on the left. When the head passes over the PES pattern, the bursts that are within range generate an analog signal (waveform) that indicates the fractional position of the head over the disk and is used as feedback to adjust the position of the head. As the term “servo wedge” suggests, the downtrack dimension of the servo sectors increases toward the OD, because the linear velocity increases from the ID to the OD. This allows servo frequency to be keep constant. However, no information about the relative position of the track (the track ID) is encoded in the PES prior art.
Published US patent application 20100165512 by Albrecht et al. (Jul. 1, 2010) describes a method for forming a master pattern for patterned media, including features to support servo patterns. Block copolymer self-assembly is used to facilitate the formation of a track pattern with narrower tracks. The tracks include regions within each servo sector where the tracks are offset radially by a fraction of a track pitch, e.g. one half track. As one example, the offset portion of servo sector is self-written with A and B patterns on alternating tracks and a non-offset portion is self-written with C and D patterns on alternating tracks. The A-D patterns are magnetized in a self-servo writing operation, wherein the write head writes a burst (e.g., typically square wave) of alternating magnetization polarities.
Published US patent application 20100128583 by Albrecht; et al. (May 27, 2010) describes a servo writing method for patterned-media magnetic recording disk that uses a special position error signal (PES) alignment pattern located in each servo sector. The servo sectors include a synchronization (sync) field and a PES field that may include burst fields (A-D). The A-B fields are shown as being radially shifted by one-half track from the dots in fields C-D. The set of radial offsets for all of the servo sectors is used to modify or fine tune the gross feedforward correction signal that is applied during the servo writing process. This enables the servowriter write head to then precisely follow a track centerline so that the discrete islands in the PES fields can be magnetized according to the desired pattern.
Published US patent application 20090166321 by Albrecht, et al. (Jul. 2, 2009) describes formation of servo patterns for magnetic media that include self-assembly structures. The servo pattern is defined through lithographic processes while the data pattern is defined by a combination of lithographic processes and self-assembly. The servo regions may each include a sync field and plurality of burst fields (A-D), which in FIG. 3 are shown as including offset portions with the islands (dots) for the A and B bursts being generally positioned one half track offset from the respective track centerline. The A and B bursts are offset one track width from each other as well as being sequentially separated.
Published US patent application 20090097160 by Yamamoto (Apr. 16, 2009) describes a magnetic recording disk with pre-patterned servo sectors, wherein each data bit is stored in a magnetically isolated data island on the disk. The servo sectors include a synchronization pattern of generally radially directed discrete magnetized marks, and first and second position error signal (PES) fields of generally radially directed discrete magnetized stripes. Each stripe in each of the two fields comprises a plurality of radially spaced discrete servo islands, each island having a radial width of approximately Tp, where Tp is the radial spacing of the track centerlines. In each field, the servo islands in alternating stripes in the along-the-track direction are shifted radially by approximately Tp. In the first PES field, the islands are centered at the midline between two adjacent track centerlines, and in the second PES field the islands are centered at a track centerline. All of the servo islands in the two PES fields have the same magnetization direction.
Published US patent application 20090168229 by Albrecht; et al. describes a method of fabricating servo sectors of a patterned storage media with two arrays of discrete islands in a servo sector. The first array defines at least two burst fields. The second array also defines at least two burst fields. The second array is formed with a track-wise offset (i.e., an offset in the radial direction) from the first array. The offset between the first array and the second array may be about a half track offset, but the amount of offset between the arrays may vary depending on design preferences. Another step of the method comprises performing a servo writing process to define the polarity of the islands in the arrays. The servo writing process is performed by circumferentially writing one or more rows of islands to define the servo pattern in the servo sector. The servo pattern may be burst fields, sync fields, etc. The servo pattern generated by the servo writing process allows a quadrature signal to be generated when a read/write head passes over a track of the patterned storage media.
In U.S. Pat. No. 6,643,082 to Karl Belser (Nov. 4, 2003) a servo sector format for patterned media is described that includes a first patterned servo timing mark, a patterned Gray code, a plurality of PES burst separators (where no data can be written), and a second patterned servo timing mark. The first patterned servo timing mark indicates the start of a servo sector. A plurality of servo burst fields are written magnetically between the plurality of PES burst separators, and are used to determine at least one position error signal. The servo sector format further includes magnetically written Gray code positioned after the second servo timing mark. The patterned Gray code is used in addressing the tracks located on the surface of a disk when the magnetically written Gray code is self-written.